Certain geologic features in shallow subsurface, such as faults or sinkholes may be hazardous to the stability of surface structures, such as mines, dams, and buildings. In order to mitigate the hazards associated with such geologic features, subterranean formations may be surveyed and imaged.
As shown in FIG. 1, imaging a subterranean formation 100 with geophones may involve sending a primary wave 102 down below the earth's surface 104. The primary wave 102 may be generated from a seismic energy source, and the primary wave 102 may be a compressional or a shear wave. The energy from the primary wave 102 may be reflected at a reflection point 106 as a secondary wave 108. The reflection may be recorded by geophones placed at a secondary wave observation point 110 at the earth's surface 104. The data collected by the geophones may then be processed to image geologic features of the subterranean formation 100. Additional geophones may be present, allowing further data collection. For example, as illustrated in FIG. 1, a geophone may be present at an additional secondary wave observation point 110′, such that a portion 102′ of the primary wave 102 may be reflected at another reflection point 106′ as an additional secondary wave 108′. Geophones used to collect measurements may be placed at a predetermined distance from a source point 112 from which the primary wave 102 is generated. A distance 114 between the source point 112 and the secondary wave observation points 110, 110′ may be approximately equal to the depth of the feature for which data is being collected. Thus, a vertical distance 116 between the source point 112 and the first reflection point 106 may be approximately equal to the distance 114 between the source point 112 and the secondary wave observation point 110. Similarly, the depth (not shown) of the additional reflection point 106′ may be approximately equal to the distance (not shown) between the source point 112 and the additional secondary wave observation point 110′. Placement of the geophones at the secondary wave observation points 110, 110′ may provide an indication of features, such as boundaries 118, 120 between geologic layers 122, 124, 126 of the subterranean formation 100. However, features such as fault 128 lying outside the reflection area may not be observed. Thus, the process may be repeated, with a new seismic generation event at a new source point and data collection at new secondary wave observation points.